NGN (Next Generation Network) is a hot research subject in the current communication field, and it adopts packet techniques such as IP (Internet Protocol) as the bearer network technology to converge fixed telecommunication network with mobile telecommunication network, and can provide more diverse multi-media services for subscribers.
The European standardization organization of TISPAN (Telecommunication and Internet Converged Services and Protocols for Advanced Networking) set up a special research group working on formulating NGN technology standards. TISPAN divides NGN architecture into a service layer and a transmission layer, and NASS (Network Attachment Subsystem) and RACS (Resource and Admission Control Subsystem) are incorporated into the transmission layer to be responsible for providing separate subscriber access management and resource control assurance functions for the upper service layer.
The functional architecture of TISPAN RACS is shown in FIG. 1. RACS associates the resource requirement of the service layer with the resource distribution of the bearer layer of the network, and mainly implements functions such as policy control, resource reservation, admission control, NAT (Network Address Translation) traversal and so on. RACS provides control service of the transmission layer for applications through a series of QoS (Quality of Service) policies so as to enable the subscriber terminal to obtain the required QoS assurance.
RACS is composed of two entities: SPDF (Service-based Policy Decision Function) and A-RACF (Access-Resource and Admission Control Function).                SPDF        
SPDF provides uniform interfaces for the application layer, shields the underlying network topology structure and specific access types and provides service-based policy control. SPDF selects a local policy according to the request of Application Function (referred to as AF in short) and maps the request to an IP QoS parameter, which is transmitted to A-RACF and BGF (Border Gateway Function) for controlling corresponding resources.                A-RACF        
A-RACF controls an access network, and has functions of admission control and network policy convergence. A-RACF receives a request from SPDF, and then achieves admission control based on the stored policy by accepting or refusing the request for transmitting resources. A-RACF obtains network attachment information and QoS profile information of the subscriber from NASS through e4 reference point (interface), thus determining available network resources according to network position information (e.g., the address of the physical node accessing the accessed subscriber), meanwhile referring to the QoS profile information of the subscriber when processing a resource distribution request.
A transmission layer also comprises two function entities: BGF (Border Gateway Function) and RCEF (Resource Control Enforcement Function).                BGF        
BGF is a packet-to-packet gateway, and can be located between an access network and a core network (for implementing core border gateway function); it can also be located between two core networks (for implementing interconnection border gateway function). BGF implements NAT traversal, gate control, QoS label, bandwidth restriction, use measurement, and resource synchronization functions under the control of SPDF.                RCEF        
RCEF carries out Layer 2/Layer 3 (L2/L3) media steam policy transmitted from A-RACF through Re reference point and implements functions including gate control, QoS label and bandwidth restriction.
RACS supports QoS resource control in two modes which are “PULL” and “PUSH” modes to adapt to different types of UEs (User Equipments).
The so called PUSH mode means that AF requests RACS for QoS resource authorization and resource reservation for a service initiated by UE; if the request can be satisfied, then RACS actively pushes a decision to a transmission entity (RCEF) to obtain corresponding transmission resources. This mode applies to all types of UEs.
The so called PULL mode means that AF requests RACS for QoS resource authorization and resource reservation for a service initiated by UE, and a transmission entity (RCEF) actively requests RACS for a decision when receiving a transmission layer QoS signaling message. This mode applies to a UE which has transmission layer QoS negotiation ability and which can explicitly request QoS resource reservation through a transmission layer signaling.
TISPAN NASS and RACS transmit messages via an e4 interface. NASS implements management for attachment of a subscriber to an access network, and its main functions comprises:                dynamically providing an IP address and other configuration parameters for a UE;        performing authentication on the access layer for the subscriber;        performing authorization of network access for the subscriber based on the subscriber service profile;        configuring an access network based on the subscriber service profile;        managing location information of the subscriber.        
When the UE nomadizes, NGN can be divided into Home NGN and Visited NGN.
To sum up, RACS and NASS are critical components of the next generation network. RACS provides transmission resource control service for applications through a series of policies, associates the resource requirement of the service layer with the resource distribution of the network bearer layer, and mainly implements policy control, resource reservation, admission control, NAT traversal and so on. NASS implements management for attachment of a subscriber to an access network, and achieves IP address distribution, authentication and authorization of transmission layer, access network configuration, position information management and so on. NASS is responsible for controlling access authentication of a terminal in mobile or nomadism scenario.
Resource and admission control in nomadism scenario is quite different from that in non-nomadism scenario, and the subscriber data of the home network may be partially open or open to the visited network. Subscriber data of the home network being partially open to the visited network means: the visited network can only obtain part of the subscriber data from the home network, in which case operations such as authentication of the subscriber can only be performed in the home network. Subscriber data of the home network being open to the visited network means that the visited network can obtain all the subscriber data from the home network, in which case operations such as authentication of the subscriber may be performed in the visited network.
Currently, there is no method in the prior art for realizing resource and admission control in PUSH mode at control layers (i.e., layers involving RACS and NASS) in nomadism scenario in NGN.